This invention relates generally to magnetic resonance imaging, and more particularly the invention relates to magnetic resonance imaging with selectively suppressed material based on spin-lattice relaxation time (T1) and spin--spin relaxation time (T2).
Magnetic resonance imaging (MRI) is a non-destructive method for the analysis of materials and represents a new approach to medical imaging. It is generally non-invasive and does not involve ionizing radiation. In very general terms, nuclear magnetic moments are excited at specific spin precession frequencies which are proportional to the local magnetic field. The radio-frequency signals resulting from the precession of the spins are received using pick up coils. By manipulating the magnetic fields, an array of signals is provided representing different regions of the volume. These are combined to produce a volumetric image of the nuclear spin density of the body.
Different atomic nuclei can have different magnetic resonance due to differing magnetogyric ratios, .gamma.. In accordance with the Larmor relationship, the angular frequency .omega..sub.0 of nuclei precession is the product of the magnetic field B.sub.0 and the magnetogyric ratio, .gamma.. This allows selective excitation and imaging of different nuclei, and by applying a magnetic gradient to the imaged object, different slices through the object can be selectively excited and imaged.
Additionally, the transverse magnetization is not only related to the specific nuclei that are excited, but also to their motional characteristics manifested by relaxation times. The spin-lattice relaxation time, T1, is equivalent to a regrowth of the longitudinal magnetization following excitation of nuclei with transverse RF magnetic fields. The spin--spin relaxation time, T2, reflects the decay time of a transverse component of free induction signals following excitation.
A variety of techniques have been developed to suppress material in image signals based on a particular physical parameter. This includes the saturation of a chemical shift species using a frequency selective excitation pulse, and the saturation of blood or other moving material via the application of a selective excitation to the blood upstream of the imaging region. Additionally, a preparatory magnetization transfer (MT) excitation has been used to reduce the longitudinal magnetization of species exhibiting greater amounts of MT phenomena. See Hu and Conolly U.S. Pat. No. 5,250,898. Additionally, the T1 and T2 relaxation times have been used in selective imaging. A relatively long preparatory low level RF pulse has been used to saturate longer T2 species. Further, a long echo time, TE, readout can be used to suppress shorter T2 species. Inversion recovery can be used to null a particular T1 species by applying a 180.degree. inversion pulse and timing the ensuing readout to occur when the T1 species is passing through its null point in the longitudinal magnetization recovery. Also, a nonselective driven equilibrium sequence (i.e., T2 weighted preparation sequence) can be used with a long echo time TE readout to suppress shorter T2 species.
The present invention is directed to selective suppression of material depending on the material's T1 and T2 relaxation times.